This invention concerns an alumina-based composite sintered material which is a sintered material comprising alumina as a main ingredient and, more in particular, it relates to a wear resistant member such as a cutting tool excellent in wear resistance and chipping resistance.

Heretofore, alumina-based composite sintered materials have been used generally as wear resistant members (wear resisting members) including cutting tools since they have excellent mechanical properties. Further, various improvement have been attempted in recent years by compounding with other ingredients for further improvement of the performance.

For example, Japanese Patent Laid-Open No. 104943/1997 proposes a technique of compounding zirconia (Zr0₂) thereby suppressing development of cracks to increase toughness and improve the chipping resistance.

However, since zirconia has low hardness and poor heat thermal impact resistance, it is not yet satisfactory in view of the wear resistance and heat resistance.

Further, Japanese Patent No. 2720093 proposes a technique of compounding titanium carbide (TiC) of high hardness with needle alumina (Al₂O₃) to increase toughness while keeping hardness to some extent, thereby improving the chipping resistance in addition to the excellent wear resistance.

However, the needle alumina not only involves a problem in view of cost since it is expensive but also tends to form gaps to the periphery of needle alumina thereby resulting in a worry that incorporation of needle alumina per se rather causes defects.

Further, since titanium carbide is less sinterable, the material has to be densified by sintering at high temperature or applying hot pressing. However, this results in a problem that the entire structure grows into coarse grains which lowers the hardness and which lacks wear resistance when sintered at high temperature, or the shape is restricted when sintered by hot pressing.

Further, a technique of re-sintering an HIP sintered material in an N₂ atmosphere to make a value N/(C+N) maximum at the surface and decrease the value from the surface to the interior is disclosed in Japanese Patent Laid-Open No. 208304/1993. However, when the HIP sintered material is re-sintered, closed pores closed during the HIP process form open pores again and the structure at the surface tends to grow into coarse grains which lowers the strength. Further, since TiN has a lower hardness and tends to be poor in the wear resistance compared with TiC, it is necessary to control the nitridation amount at the surface in order to maintain the wear resistance, but the HIP sintered material is substantially densified which results in a problem that control of the amount of N₂ introduced at the surface by resintering is extremely difficult.

EP-A-0 926 108 discloses a ceramic cutting tool comprising in its essential part a ceramic material comprising a titanium carbonitride-based hard phase and an alumina-based phase, wherein the ceramic material satisfies the following relationships: 10 wt% ≤ WAl₂O₃ ≤ 45 wt%; 51 wt% ≤ WTiCN ≤ 89.5 wt%; and 96 wt% ≤ WAl₂O₃ + WTiCN ≤ 99.5.wt% wherein WAl₂O₃ indicates the weight content of aluminium as calculated in terms of Al₂O₃; and WTiCN indicates the weight content of said titanium carbonitride hard phase, with the proviso that WTiCN is defined by WTi + WC + WN in which WTi, WC and WN are the weight content of titanium component, carbon component and nitrogen component, respectively..

This invention has been made in view of the foregoing problems and an object thereof is to improve the defects of alumina-based composite sintered materials and obtain an alumina-based sintered material and a wear resistant member not only excellent in the wear resistance but also excellent in the chipping resistance.

Accordingly, the present invention provides an alumina-based composite sintered material comprising alumina as a main ingredient and containing at least one carbonitridation product of at least one element of groups IVa, Va and VIa of the periodical table and/or at least one carbonitridation product comprising solid solution of two or more elements of groups IVa, Va and VIa of the periodical table, wherein the content of nitrogen solid solute in said carbonitridation product increases from the interior to the surface of the sintered material, the Vickers hardness at the surface of the sintered material is 19.5 GPa or more, the carbonitridation product comprises grains having a titanium carbide centre covered with a solid solute containing nitrogen, the content of the carbonitridation product in the alumina-based sintered material is 10 to 40% by weight, and the content of nitrogen solid solute in the carbonitridation product is 40 atm% or less, wherein the value of the nitrogen content in atm% represents the proportion of carbon atoms that have been substituted by nitrogen atoms.

It is considered, for example, as shown in Fig. 1(a) of the accompanying drawings, that titanium carbide (TiC) grains are intruded between alumina grains in the alumina-based composite sintered material according to this invention. Nitrogen (N) is dissolved as a solid solute in titanium carbide to form solid solution (Ti(C, N)) of a carbonitridation product. The content of nitrogen solid solute in titanium carbide is expressed by the size of the annulus around the central circle representing TiC.

As can be seen also from Fig. 1(a), the content of nitrogen solid solute in the carbide increases from the interior to the surface of the sintered material. On the other hand, in the existent product , shown in Fig. 1(b), the content of nitrogen solid solute in the carbide is substantially identical between the interior and the surface of the sintered material.

As described above, since the content of nitrogen solid solute in the carbonitridation product increases from the interior to the surface of the sintered material, that is, the content of nitrogen solid solute in the carbonitridation product is greater in the surface than in the sintered material in this invention, sinterability is high to enable densification at low temperature. Therefore, a sintered material of high hardness and high strength having a fine structure, that is, an alumina-based composite sintered material having excellent wear resistance and chipping resistance is obtained.

Further, in this invention, since the Vickers hardness at the surface of the sintered material is 19.5 GPa or more by the constitution for the content of nitrogen solid solute described above in this invention, this can provide sufficient wear resistance, for example, as wear resistant members such as a cutting tools.

The content of the carbonitridation product (based on the entire sintered material) is defined as 10 to 40% by weight, because particles of alumina as the main ingredient do not grow into coarse grains and do not lower the strength and the hardness of the sintered material when it is 10% by weight or more, and the sinterability is not lowered and densification at low temperature where the entire structure becomes fine is possible when it is 40% by weight or less.

Then, a more preferred effect is obtained within a range of 25 to 35% by weight.

The content of nitrogen solid solute in the carbonitridation product is defined as 40 atm% or less (naturally exceeding 0 atm%), because the hardness of the carbonitridation product is not lowered and lowering of strength and hardness by the promotion of grain growth of the main ingredient alumina can be prevented to suppress growth into coarse grains at the amount of 40 atm% or less.

Then, a more preferred effect is obtained within a range of 20 to 30 atm%.

The effect by the definition for the content of nitrogen solid solute is remarkably obtained at the surface of the alumina-based composite sintered material in contact with a work, for example, in a case of using a cutting tool.

The alumina-based composite sintered material referred to herein is a sintered material comprising alumina as a main ingredient (for example, 60% by weight or more), compounded with addition of other ingredients. Further, the load used for measuring the Vickers hardness is 1.0 kgw.

One or more carbonitridation products of the groups IVa, Va and VIa of the periodical table can include, for example, Ti(C, N), Zr(C, N) for group IVa, V(C, N), Nb(C, N), Ta(C, N) for Va group and VIa Cr3(C, N) for VIa group.

Further, at least two carbonitridation products of solid solutions of the groups IVa, Va and VIa of the periodical table can include, for example, (Ti, W)(C, N) and (W, Ta, Nb)(C, N).

The alumina-based composite sintered material according to this invention contains at least one carbonitridation products of the groups IVa, Va and VIa of the periodical table and/or at least two carbonitridation products of solid solutions of the groups IVa, Va and VIa of the periodical table, and it may contain both of the carbonitridation products.

Preferably the element belonging to the groups IVa, Va and VIa of the periodical table is one or more of Ti, V and Zr.

This exemplifies metal elements contained in carbonitridation products of solid solutions of the group IVa, Va and VIa of the periodical table.

By the use of the metal elements, the alumina-based composite sintered material having excellent properties described above can be used.

One or more carbonitridation products of Ti, V and Zr can include, for example, Ti(C, N), V(C, N) and Zr(C, N). Further, two or more carbonitridation products of solid solutions of Ti, V and Zr can include, for example, (Ti, V)(C, N) and (Ti, Zr)(C, N).

The content of nitrogen solid solute in the carbonitridation product is defined as 40 atm% or less (naturally exceeding 0 atm%), because the hardness of the carbonitridation product is not lowered and lowering of strength and hardness by

The carbonitridation product includes one or more carbonitridation products of the groups IVa, Va and VIa of the periodical table and/or two or more carbonitridation products of solid solutions of the groups IVa, Va and VIa of the periodical table.

Preferably a hard layer, formed by coating with a hard material, is provided on the surface of the sintered material.

TiC, TiN, TiCN and/or Al₂O₃ can be used as the hard material, for instance, to thereby further improve the wear resistance.

As the coating method, PVD (Physical Evaporation Deposition method) or CVD (Chemical Vapor Deposition method) can be used, for instance.

The invention further provides a wear resistant member comprising an alumina-based composite sintered material as defined above.

The wear resistant member according to this invention is composed of an alumina-based composite sintered material described above. Accordingly, it is excellent in wear resistance and chipping resistance.

The invention further provides that the wear resistant member comprising the wear resistant material is a cutting tool.

Accordingly, the cutting tool is excellent in wear resistance and chipping resistance.

The wear resistant member can include, in addition to the cutting tool, for example, bearings, dies, press molds, chain saw blades and sliding members of engines.

Also disclosed is a method of manufacturing an alumina-based composite sintered material comprising alumina as a main ingredient, wherein nitrogen is dissolved as a solid solute in at least one carbide to form a carbonitridation product, comprising the steps of: forming a powder molded product by compounding one or more carbides of at least one element of groups IVa, Va and VIa of the periodical table with the alumina as a main ingredient; and sintering said molded product while controlling an Ar/N₂ gas mixing ratio and controlling a temperature elevation rate for the sintering step, such that the content of nitrogen solid solute in the carbonitridation product increases from the interior to the surface of the sintered material, the Vickers hardness at the surface of the sintered material is 19.5 GPa or more, the carbonitridation product comprises grains having a titanium carbide centre covered with a solid solute containing nitrogen.

According to this method, sinterability can be improved and the material can be densified at low temperature by further increasing the content of nitrogen solid solute near the surface of the sintered material while maintaining the hardness of the starting carbide material by nitriding the surface of the starting carbides material. This can provide a sintered material of high hardness and high strength having a fine structure.

Accordingly, when they are used as the wear resistant member, excellent wear resistance and chipping resistance can be obtained.

An HIP (Hot Isostatic Pressing) treatment may optionally be applied for increasing the density.

The method is based on the following findings. An Ar/N2 gas mixing ratio and a temperature elevation rate are defined within specified ranges upon sintering of a powder molded product comprising alumina as a main ingredient, compounded with carbides of the groups IVa, Va and VIa of the periodical table. This dissloves nitrogen as a solid solute into the carbides to form carbonitridation products and increases the content of nitrogen solid solute in the carbonitridation product from the interior to the surface of the sintered material. It is considered that grain boundary diffusion of alumina and carbide is promoted as the content of nitrogen solid solute increases. Since this improves the sinterability to enable densification at low temperature, it is possible to obtain a sintered material of high hardness and high strength having a fine structure.

More specifically, by nitriding the surface of the starting carbide material, a sintered material of high hardness and high strength having a fine structure is obtained by further improving the sinterability to increase the density near the surface of the sintered material while maintaining the hardness of the starting carbide material. Then, it has been found, as a result of using them for the wear resistant member, that excellent wear resistance and chipping resistance can be attained.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

• Fig. 1 schematically shows the structure of a sintered material in which (a) is an explanatory view showing an example of a product of this invention and (b) is an explanatory view showing an example of an existent product;
• Fig. 2 shows a cutting tool in which (a) is a plan view and (b) is a front elevational view;
• Fig. 3 is a graph showing the result of analysis by XPS for the content of nitrogen solid solute;
• Fig. 4 is a plan view showing a work to be cut; and
• Fig. 5 is a graph showing the result of a cutting test.

Now, examples for the mode of practicing an alumina-based composite sintered material and a wear resistant member, as well as a method of manufacturing an alumina-based composite sintered material according to this invention are to be explained below.

1. (a) Firstly, a cutting tool composed of an alumina-based composite sintered material of this example will be explained.

As shown in Fig. 2, a cutting tool 1 of this example has a substantially trigonal plate-like shape (for example, TNGN 333 shape according to JIS Standard number JIS B4121 of the year 1985), which is a so-called ceramics chip used being attached to a top end of a holder.

The cutting tool 1 is composed of an alumina-based composite sintered material formed by sintering a material comprising alumina (Al₂O₃) as a main ingredient with addition of other ingredients in order to improve, for example, wear resistance or sinterability.

Other ingredients to be compounded with alumina can include at least one carbonitridation products of the groups IVa, Va and VIa of the periodical table (for example, carbonitridation product of Ti, V, Zr) or at least two carbonitridation products of solid solutions of the groups IVa, Va and VIa of the periodical table (for example, carbonitridation products of solid solutions of Ti, V and Zr). Alternatively, it may be both of the above carbonitridation products.

Particularly, in this example, the content of nitrogen solid solute in the carbonitridation product increases in the alumina-based composite sintered material from the interior to the surface of the sintered material and, further, the surface of the sintered material has a high hardness of 19.5 GPa or more Vickers hardness.

Further, in a more preferred embodiment of the example, the content of the carbonitridation product of the groups IVa, Va and VIa of the periodical table is within a range from 10 to 40% by weight and, further, the content of nitrogen solid solute in the carbonitridation product of the groups IVa, Va and VIa of the periodical table is 40 atm% or less.

For improving the wear resistance or the like on the surface of the cutting tool 1, a hard layer made of a hard material such as TiC may be disposed thereon.

(b) Next, an alumina-based composite sintered material and manufacturing method of a cutting tool 1 constituted therewith will be explained.

At first, a starting material was formulated by adding carbides as the compositing material to be compounded at a ratio shown in the following Table 1 (Table for the formulated composition) to commercially available starting alumina material and the material is mixed and pulverized by an attritor using acetone as a medium and formed into a slurry. Composition example Composite hard carbide (wt%) Balance (Al₂O₃) A TiC 5 B TiC 10 C TiC 20 D TiC 40 E TiC 50 F TiC 25, TaC 10 G TiC 25, ZrC 10 H TiC 25, WC 10 I TiC_0.3N_0.7 20, TaC 10 J TiC_0.5N_0.5 20, TaC 10

Then, the slurry was dried and a binder, for example, paraffin was added to the resultant powder mixture, which was subjected to 1-axis pressing to obtain a molded product (molded product of a trigonal plate shape).

Then, the resultant molded product, after a binder removing treatment in vacuum at 800°C, was preliminarily sintered in an Ar/N₂ mixed gas stream at 1600 - 1700°C to such an extent that the surface was densified.

Then, an HIP treatment was applied to the preliminarily sintered product in an atmosphere of an Ar gas at a pressure of 130 to 160 MPa at a sintering temperature of 1450 to 1500°C to prepare an alumina-based composite sintered material substantially of the shape as shown in Fig. 2.

Accordingly, by grinding the surface of the alumina-based composite sintered material entirely by about 0.2 mm, a cutting tool 1 of the shape shown in Fig. 2 was obtained.

Further, in the same manner, sintered material and cutting tools of comparative example were also manufactured.

In table 2, specimens Nos. 2 - 4, 8, 9, 11, 13 - 15 and 17 - 19 are examples within the scope of this invention and others are comparative examples out of the scope of this invention. Further, a TiN coating was applied to specimen No. 14 by PVD on the surface of the alumina-based composite sintered material.

(c) Next, description is to be made for experimental examples to the alumina-based composite sintered material and the cutting tool obtained by the manufacturing method described above.

1. <1> The content of nitrogen solid solute in Ti(C, N) as typical examples for those of the groups IVa, Va and VIa of the periodical table was measured by XPS (X-ray Photoelectron Spectroscopy) of the alumina-based composite sintered materials of specimens Nos. 1 - 21 both for the surface and the interior. The result is also shown in the following Table 2.
   The measuring position at the interior is at 2.5 mm depth from the surface along the direction of the plate thickness in the axial center of a sintered material of a trigonal columnar shape.
2. <2> Further, the content of nitrogen solid solute in Ti(C, N) was measured by XPS of sample Nos. 1 - 21 of alumina-based composite sintered materials from the surface to the interior. The result is shown together with the distance from the surface in Fig. 3.
3. <3> Further, the Vickers hardness Hv at the surface of the sintered materials was measured under a load of 1.0 kgw. The result is also shown in the following Table 2.
4. <4> Further, a cutting test was conducted under the following conditions by using a cutting tool prepared by grinding the surface of the sintered material by about 0.2 mm into the TNGN 333 shape.

Work to be cut: JIS SCM415 (JIS number JIS G4105 of 1979) Cutting speed V: 250 m/min Feed speed f: 0.16 mm/rev Notch d: 0.2 mm Dry/wet: DRY

In the cutting test, a circular columnar portion, as shown in Fig. 4, of a work having some grooves of predetermined width (6 mm) and depth (7 mm) with interval (22 mm) was cut. For the judgement of the cutting life, it was evaluated when the wear amount at the relief surface of the cutting tool reached 0.05 mm or more, or when chipping occurred.

The result is also shown in the following Table 2 and Fig. 5. Specimen No Formulated composition Sintering temp. (°C) Ar/N₂ gas mixing ratio Temperature elevation rate °C/min Content of N solid solute in TiC (atm%) Hv at surface used (GPa) Working distance till end of cutting life (m) Surface Interior Comparison 1 A 1650 10/0.5 5 31 13 17.5 123 Invention 2 B ↑ ↑ ↑ 37 16 198 513 3 C ↑ ↑ ↑ 34 16 20.5 492 4 D ↑ ↑ ↑ 38 17 19.6 451 Comparison 5 E ↑ ↑ ↑ 38 14 18.0 instant chipping 6 C ↑ 10/10 ↑ 98 98 16.4 103 7 C 1700 10/5 ↑ 73 70 17.6 144 Invention 8 C ↑ 10/1 ↑ 32 17 20.0 431 9 C ↑ 10/0.5 ↑ 2 0 21.0 431 Comparison 10 C ↑ 10/1 ↑ 82 83 184 103 Invention 11 C ↑ 10/1 10 14 2 20.3 595 Companson 12 C ↑ ↑ 20 21 3 18.5 life unstable Invention 13 F ↑ ↑ 5 25 16 212 656 14 F ↑ ↑ ↑ 25 16 21.2 718 15 H ↑ ↑ ↑ 22 17 20.8 595 Comparison 16 J ↑ ↑ ↑ 84 78 17.7 103 Invention 17 C ↑ ↑ 3 28 21 20.0 574 18 G ↑ ↑ ↑ 27 22 21.0 595 19 H ↑ ↑ ↑ 28 23 20.6 554 Companson 20 I ↑ ↑ ↑ 94 91 17.4 41 21 J ↑ ↑ ↑ 98 77 183 82

(d) As apparent from the Table 2, in the alumina-based composite sintered materials within the scope of this invention (specimen Nos. 2 - 4, 8, 9, 11, 13- 15, 17 - 19), the content of nitrogen solid solute in Ti(C, N) is greater at the surface than in the interior of the sintered material (for example, gradually increases toward the surface as shown in Fig. 3) and the Vickers hardness on the surface of the sintered material is 19 MPa or more.

Accordingly, since the cutting tools of the examples according to this invention are excellent in the wear resistance, the cutting distance is long and working life is long. In addition, since they have high strength, chipping occurs less upon cutting, which is favorable.

Further, in the examples of this invention, B, C, D, F, G and H are used as the formulated composition and the content of carbonitridation products of the groups IVa, Va and VIa of the periodical table is 10 to 40% by weight. Further, in the examples of this invention, the content of nitrogen solid solute in the carbonitridation product of the groups IVa, Va and VIa of the periodical table is 40 atm% or less.

Particularly, specimen No. 14 having a hard layer formed by coating a hard material on the surface of the sintered material has the longest working distance of 715 m and most excellent wear resistance as shown in Fig. 5.

On the contrary, since comparative examples do not satisfy both of the conditions for the change of the content of nitrogen in solid solution and Vickers hardness, the working distance is as short as 144 mm or less and the wear resistance is not sufficient. Particularly, those with the content of nitrogen solid solute at the surface of the sintered materials exceeding 40 atm% have low wear resistance. Further, some of comparative examples result in chipping in the course of working (specimen No. 5) or unstable life (specimen No. 12) and are not satisfactory also in view of the strength.

Further, this invention is not restricted at all to the examples described above and it will be apparent that it can be practiced in various modes within a range not departing the scope of the invention.

For example, while the cutting tool is mentioned as an example of the wear resistance member in the examples, the alumina-based composite sintered material of this invention is not restricted to such cutting tool but is usable also, for example, as various kinds of wear resistant members such as bearings, dies, press dies, chain saw blades and sliding members of engines. Furthermore, in addition to the wear resistance member, it is usable, for example, in applications requiring structural materials at high temperature.

As described above specifically, according to this invention, high hardness and strength can be obtained compared with existent alumina-based composite sintered materials and, when they are used as wear resistant members, such as cutting tools, they can provide excellent wear resistance and chipping resistance and the life can be extended remarkably.